1. Field of the Invention
The present invention relates to a laser gyroscope, and more particularly, to a fiber-optic laser gyroscope having phase sensitive detecting means which can reduce rotation rate measurement errors.
2. Description of the Prior Art
FIG. 1 shows a schematic diagram of a conventional fiber laser gyroscope. Referring to FIG. 1, the conventional fiber laser gyroscope is constructed with a laser resonator having an optical amplifier 10 positioned midway, a mirror 20 attached at one end thereof, and a fiber-optic loop reflector which comprises a directional coupler 30 and a fiber-optic coil 40 connected at the other end thereof via a directional coupler 30, which performs role of a mirror and a rotation sensing element as well. The fiber-optic loop reflector is provided with a phase modulator 50. When mode-locking occurs in the laser resonator by a function signal of a function generator applied to the phase modulator 50, an output 70 from another directional coupler 34 has the form of pulse train with its time interval change depending upon rotation.
Referring back to FIG. 1, the operation of the above conventional fiber laser gyroscope will be described.
When a pumping light 80 is transmitted into the optical amplifier 10, the light is absorbed by the optical amplifier 10, and light having new wavelength is oscillated. The light oscillated as such is input into the directional coupler 30 of the fiber-optic loop reflector and divided into two light beams, and the separated beams propagate in opposite directions around the fiber-optic coil 40. Then, the two beams are recombined at the directional coupler 30 to interfere with each other. Then, a phase difference between two interfering light beams has a feature of being linearly proportional to the rotation rate due to Sagnac effect. If a function signal which has a frequency ƒm corresponding to the longitudinal mode spacing of this laser resonator is applied to the phase modulator 50, the phase difference between two light beams propagating around the fiber-optic coil 40 in opposite directions is modulated, thereby a mode-locking occuring among longitudinal modes of the laser resonator.
FIG. 2A and FIG. 2B show pulse trains observed respectively with an oscilloscope, which are outputs when the conventional fiber laser gyroscope is in static and rotational state. Mode-locked pulses 100 are generated such that two periodic pulses are generated in respect to one phase modulation 110 period (Tm=1/ƒm). When the fiber-optic loop reflector is stationary, mode-locked pulses 100 are generated in an equal spacing, as illustrated in FIG. 2A. When the fiber-optic loop reflector rotates, however, two mode-locked pulses, generated in respect to one phase modulation period, are shifted in opposite directions with each other, as shown in FIG. 2B, therefore the mode-locked pulses 100 no longer have an equal spacing.
Then, the shifts of two pulses due to rotation have same absolute value, but are opposite in direction. The time interval between two pulses shifted in opposite directions with each other changes depending upon a rotation rate, in which the changing values (xcex94T) can be represented by the following equation 1:                              Δ          ⁢                      xe2x80x83                    ⁢          T                =                                            T              m                        π                    ⁢                      xe2x80x83                    ⁢                                    sin                              -                1                                      ⁡                          (                                                φ                  R                                                  φ                  m                                            )                                                          (                  equation          ⁢                      xe2x80x83                    ⁢          1                )            
where, Tm is a phase difference modulation period and has a relationship of Tm=1/ƒm to the frequency ƒm of the function signal. xcfx86m is a depth of phase difference modulation, and xcfx86R is a Sagnac phase shift due to rotation, which is a phase difference between two light beams propagating around the fiber-optic coil 40 in opposite directions with each other, and has a relationship as the following equation 2:                               φ          R                =                                            2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢              LD                                      λ              ⁢                              xe2x80x83                            ⁢              c                                ⁢          Ω                                    (equation  2)            
where, L is a length of the entire optical fiber of the fiber-optic loop reflector which is a fiber-optic rotation sensing element, D is a diameter of the fiber-optic loop reflector, xcex is a frequency of the propagating light, c is the light velocity in vacuum, and xcexa9 is a rotational angular velocity of the rotation sensing element, respectively. That is, if the fiber-optic rotation sensing element is rotating in a rotational angular velocity xcexa9, a Sagnac phase shift (xcfx86R) is generated by equation 2, with the result that the time interval between two pulses changes by xcex94T as in equation 1. And, if the fiber-optic rotation sensing element rotates in the opposite direction, the sign of xcfx86R becomes opposite, and the sign of xcex94T also becomes opposite. That is, the displacement of the pulse becomes opposite. Therefore, the rotation rate can be measured by reading temporal pulse displacement directly on the time axis.
Up to now, signal processing has been carried out by directly measuring the variation of time interval due to the temporal pulse displacement on the time axis using an electrical counter. The time interval measurement by the electrical counter is carried out on the basis of a specific trigger level. As shown in FIG. 3A, counting is started at the moment when the intensity of pulse signal is at least same as the predetermined trigger level 200, and stopped at the next moment thereof, in which the counted time therealong is a time interval between two pulses. However, the method is operated by a digital counter and includes counting error of maximum 1 digit.
Generally, in a fiber laser gyroscope, peak intensities of two pulses are different from each other due to the gain competition between two pulses. If the intensities of two pulses are different from each other, the above conventional time interval measurement method induces measurement errors by xcex4t, as shown in FIG. 3B. Therefore, the time interval measurement method is not appropriate as a signal processing method for a fiber laser gyroscope with high accuracy.
Therefore, it is an object of the present invention to provide a fiber laser gyroscope comprising phase sensitive detecting means as a signal processing method which eliminates errors due to the relative intensity difference between two pulses.
The fiber laser gyroscope of the present invention comprises: a laser resonator having a fiber-optic loop reflector at one end and a mirror at the other end thereof, the laser resonator including optical amplifying means and optical pumping means provided therein; phase modulating means, located in the fiber-optic loop reflector, for producing an optical pulse train; means for detecting the optical pulse train from the laser resonator to convert it into an electrical signal; and phase sensitive detecting means for extracting from the converted electrical signal a component having a frequency which is the same as or an integral multiple of that of a phase-modulated signal and having the same phase as the phase-modulated signal, and detecting from the extracted component the phase difference between two light beams passing through the fiber-optic loop reflector in opposite directions with each other.
Then, the phase sensitive detecting means can preferably extract the same frequency component as the phase-modulated signal from the converted electrical signal, and more preferably extract a direct current component together with the frequency component from the electrical signal.
In the gyroscope of the present invention, all optical fibers including the fiber-optic loop reflector are preferably comprised of single mode optical fibers or single mode polarization maintaining optical fibers. Furthermore, the phase modulating means may be selected from the group consisting of an optical fiber phase modulator, an integrated optic device where a directional coupler and a phase modulator are integrated, and an integrated optic device where a directional coupler, a phase modulator and a polarizer are integrated.
It is preferred that the frequency of the phase-modulated signal generated by the phase modulating means has a value differing from a longitudinal mode spacing of the laser resonator within a range of xc2x110% of the spacing or has an integral multiple of a value within the range, and the phase-modulated signal has a sine wave form or triangular wave form, and more preferably the frequency of the phase-modulated signal has the same value as a longitudinal mode spacing of the laser resonator or has an integral multiple value of the mode spacing.
Furthermore, the optical amplifier may preferably be selected from the group consisting of a rare-earth doped optical fiber amplifier, a semiconductor optical amplifier, and a semiconductor optical amplifier with an anti-reflection coating formed on one side thereof, and in particular, an erbium doped optical fiber amplifier is preferred when a rare-earth doped optical fiber amplifier is utilized.
A polarizer is preferably disposed between the mirror and the fiber-optic loop reflector, and more preferably between the optical amplifier and the fiber-optic loop reflector. The polarizer is preferably comprised of a fiber-optic polarizer or an integrated optic device where a directional coupler, a phase modulator and a polarizer are incorporated.
It is preferred that the mirror is a planar mirror, or a mirror having a wavelength selectivity such as Bragg gratings. A directional coupler can be inserted between the mirror and the fiber-optic loop reflector for extracting optical output signal of the laser resonator.